NEWS OF THE WEEK
EYEING ELECTRONS MOLECULAR DYNAMICS: Laser pulse enables tracking of electron distribution in reactions
B
Y USING LASER PULSES to align molecules
for just a few picoseconds, researchers can now study how electron rearrangement influences chemical reactions, reports a group led by Albert Stolow of the Canadian National Research Council’s Steacie Institute for Molecular Sciences (Science 2009, 323, 1464). Scientists have used femtosecond spectroscopy for years to follow atoms in chemical reactions. LINE UP A laser pulse pushes CS2 molecules The random orientainto alignment long enough to track how tion of molecules in the electron distribution changes as they ionize gas phase, however, can and dissociate. C is gray and S is yellow. make it difficult to tease out what’s happening Pump/probe photoionization to the electrons in such Alignment laser pulse reactions. For example, when a molecule is ionized, scientists would like to use the energy and direction of the 0 ps 75 ps 76 ps emitted electron to de-
© 20 09 SCIENCE
After polyurethane is scratched (top), UV light repairs the damage over the next 30 minutes (center and bottom).
COATING FIXES ITS OWN SCRATCHES MATERIALS: UV light helps replace broken cross-links in polyurethane
I
MAGINE YOU ACCIDENTALLY scratch your car’s
paint when you leave your garage in the morning. Now picture that paint repairing itself with the help of sunshine during your commute to work. That sort of application is the long-term goal of research that’s just been reported by polymer scientists Marek W. Urban and Biswajit Ghosh of the University of Southern Mississippi (Science 2009, 323, 1458). The paper describes a new self-repair approach for a polyurethane coating, says Nancy R. Sottos, a materials scientist at the University of Illinois, Urbana-Champaign. Several self-healing compounds already exist. For example, Sottos and others have designed materials that release embedded healing reactants when cracked. Other researchers have produced materials in which “the crack-mending process is initiated by an external thermal-, mechanical-, chemical-, or photo-induced stimulus,” she says. WWW.CEN-ONLINE.ORG
8
termine the shape of the molecular orbital from which it came. But if molecules are oriented randomly, the emission directions are blurred. Separately, scientists had also previously used ultrashort laser pulses to align molecules temporarily for optical or mass spectrometry experiments. Stolow and colleagues now take that one step further to combine alignment with pump/probe femtosecond laser spectroscopy for photoelectron detection. “The clever extension in this report to use photoelectron detection is a welcome contribution,” says Ahmed H. Zewail, a professor of chemical physics at California Institute of Technology. Zewail received the Nobel Prize in Chemistry in 1999 for pioneering the use of femtosecond spectroscopy to study chemical reactions. Stolow and colleagues demonstrated the method by using a femtosecond laser pulse to push molecules of carbon disulfide (CS2) into alignment. The molecules stay put for just a few picoseconds, but that’s long enough to use time-resolved photoelectron spectroscopy (TRPES) to ionize the aligned molecules and examine what happens as they dissociate. Stolow likens the power of TRPES with alignment versus without it to that of singlecrystal X-ray crystallography versus powder diffraction. “You can see the angular distribution of electrons changing and directly relate it to the way the molecular orbitals are changing during a chemical reaction,” Stolow says. “Now we can see not just how atoms are moving but how electrons are arranging in concert with the atoms.”—JYLLIAN KEMSLEY
The first photochemical repair of a polymeric material was reported by a Korean team (Chem. Mater. 2004, 16, 3982). The researchers produced their polymeric film by irradiating cinnamate monomers with UV light to form cyclobutane cross-links. Any cross-links subsequently broken by cracking could be fixed with another dose of UV light. Urban says his own work differs by incorporating a minimal amount of a self-healing additive into polyurethane. He believes his technique could be broadly applicable because the additive could be incorporated into many other types of coating materials. The researchers synthesized the additive by linking chitosan, a polysaccharide derived from crab and shrimp shells, to oxetanes, which are four-membered rings containing three carbon atoms and an oxygen. The additive also covalently bonds to the urethane polymers. When a scratch damages the material, exposure to UV light initiates a free-radical process that cross-links broken chitosan and oxetane fragments, repairing the damage in less than an hour. So far, Urban and Ghosh have demonstrated the technique with microscopic scratches that are about 10 μm wide. Further work will be needed to show the strength of the repaired material and whether the technique can handle macroscopic scratches.—SOPHIE ROVNER
MARCH 16, 2009
